The load and frequency control of a multi-area interconnected power system is studied. In this problem, the system is assumed to be subject to unknown constant disturbances, and it is desired to obtain, if possible, robust decentralized controllers so that the frequency and tieline/net-area power flow of the power system are regulated. The problem is solved by using some structural results recently obtained in decentralized control, in conjunction with a parameter optimization method which minimizes the dominant eigenvalue of the closed-loop system. A class of minimum order robust decentralized controllers which solves this general multi-area load and frequency control problem is obtained. Application of these results is then made to solve the load and frequency control problem for a power system consisting of nine synchronous machines (described by a 119th-order system). It is shown that the load and frequency controller obtained in this case is not likely to be significantly improved by using more complex controllers; in particular, it is shown that the conventional controller, used in regulating the net-area power flow of a system, is not likely to be significantly improved upon by using more complex controllers.